In vivo blood spectrometry

ABSTRACT

A process and apparatus for determining the arterial and venous oxygenation of blood in vivo with improved precision. The optical properties of tissue are measured by determination of differential and total attenuations of light at a set of wavelengths. By choosing distinct wavelengths and using the measured attenuations, the influence of variables such as light scattering, absorption and other optical tissue properties is canceled out or minimized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process and apparatus for increasing the accuracy of optical in vivo measurements of blood constituents in tissue, such as arterial oxygenation.

2. Description of Related Art

A standard method to measure the arterial oxygenation of blood is known as pulse oximetry.

Pulse oximeters function on the basis that at differing wavelengths, blood attenuates light very differently depending upon the level of oxygenation. Pulse waves starting from the heart cause in the arterial blood vessel system a periodic fluctuation in the arterial blood content in the tissue. As a consequence, a periodic change in the light absorption (FIG. 1) can be registered between the light transmitter, whose radiation passes through the tissue, and the receivers, which are integrated in a pulse oximetry sensor. The evaluation of the sensor signals is normally carried out at light wavelengths of w1=660 and w2=940 nm by calculating the differential change of light absorption at times t1 and t2. It is possible to create a measured variable R which is obtained in the following manner or in a similar manner:

$\begin{matrix} {{{Rw}\; 1},{{w\; 2} = \frac{{\ln\left( {{I\; t\; 1},{w\; 1}} \right)} - {\ln\left( {{I\; t\; 2},{w\; 1}} \right)}}{{\ln\left( {{I\; t\; 1},{w\; 2}} \right)} - {\ln\left( {{I\; t\; 2},{w\; 2}} \right)}}}} & (1) \end{matrix}$

The light intensities described in the formula represent the light intensities received in the receiver of the sensors used in pulse oximetry. The measured variable R serves as a measurement for the oxygen saturation. The formation of a quotient in order to form the measured variable is intended to compensate for any possible influences the haemoglobin content of the tissue, the pigmentation of the skin or the pilosity may have on the measurement of the oxygen saturation of arterial blood. The difference of the light attenuations at a minimum and maximum value is the delta of the light attenuations for each of both wavelengths.

Measuring oxygen saturation of arterial blood in the tissue in a range of 70 to 100% using light of wavelength 940 nm and 660 nm most often produces for one single application site sufficiently accurate measured values. However, in order to measure lower oxygen saturation of arterial blood it is necessary to assume a strong influence on the measured variable R in particular caused by perfusion (i.e. blood content) (see: IEEE; Photon Diffusion Analysis of the Effects of Multiple Scattering on Pulse Oximetry by J. M. Schmitt; 1991) and other optical parameters of tissue.

Rall, U.S. Pat. No. 5,529,064, describes a fetal pulse oximetry sensor. For this kind of application, a higher measurement precision is desirable because a fetus has a physiological lower oxygenation than adult human beings and measurement error of SaO₂ increases at low oxygenations.

U.S. Pat. No. 6,226,540 to Bernreuter, incorporated by reference herein, improves the precision of pulse oximetry. However, in order to measure on different body sites with the same high resolution for the arterial oxygenation, additional precision to measure optical tissue properties is necessary. Another problem is that pulse oximetry alone does not provide sufficient diagnostic information to monitor critically ill patients (See: When Pulse Oximetry Monitoring of the Critically Ill is Not Enough by Brian F. Keogh in Anesth Analg (2002), 94:96-99).

Because of this it would be highly desirable to be able to additionally measure the mixed venous oxygenation of blood SvO₂. Methods to measure SvO₂ with NIR were described by Jöbsis in U.S. Pat. No. 4,223,680 and by Hirano et al in U.S. Pat. No. 5,057,695. A problem of those disclosed solutions is that hair, dirt or other optically non-transparent material on the surface of tissue can influence the measured results for SvO₂.

To measure the metabolism of blood oxygenation, Anderson et al in U.S. Pat. No. 5,879,294 disclose an instrument in which the second derivative of the light spectrum used delivers information about the oxygenation. Hereby, the influence of light scattering in tissue is minimized, which can result in higher measurement precision. A disadvantage of this solution is that the calibration of the optical instruments is complicated and expensive, which makes it impractical to use such devices for sports activity applications, where light weight wearable devices would be of interest. Similar problems are known for frequency domain spectroscopy disclosed for example in Gratton, U.S. Pat. No. 4,840,485. Oximetry devices, which are described in the present specification and which simply measure light attenuations of tissue at different wavelengths, are more feasible, flexible and reliable in practice than complex time resolved methods.

SUMMARY OF THE INVENTION

Accordingly, several objects and advantages of the invention are:

a) to provide a device that measures the arterial oxygenation of blood in tissue at a certain application site with improved precision;

b) to provide a device that measures the arterial oxygenation blood in tissue at different application sites with improved precision;

c) to provide a device that measures the mixed venous or venous oxygenation blood in tissue with improved precision;

d) to provide a device that measures the mixed venous or venous and arterial oxygenation blood in tissue with improved precision with only one sensor;

e) to provide a device that measures the mixed venous or venous oxygenation blood in tissue with improved precision without complicated empirical calibration;

f) to provide an inexpensive device that measures the mixed venous or venous oxygenation blood in tissue with improved precision;

g) to provide an inexpensive device that can directly measure oxygen extraction of tissue at the application site; and

h) to provide an inexpensive, wearable device that measures oxygenation of tissue.

There are various fields of application where the invention can be used with benefit. For example for sports activity applications, a light weight, small and inexpensive device to track the oxygen metabolism would be of interest.

Critically ill persons would benefit by continuous and more detailed diagnostic information of their physiological condition.

Newborns would benefit from better care if arterial oxygenation could be measured e.g. on the back instead on the feet where unintentional alarms more often occur due to motion effects. A higher precision of pulse oximetry could improve ventilation of newborns, and precision of fetal pulse oximetry where a high resolution of the arterial oxygenation is needed, could be improved as well (See U.S. Pat. No. 6,226,540).

In accordance with invention, a device utilizes a combination of light emitters and detectors with:

a light wavelength combination with more than two wavelengths, where the peak spectrum of a third wavelength is about the geometric mean value of the first and second wavelengths;

multiple detectors and emitters which eliminate influences on calibration by subtracting and adding measured light attenuations;

a model-based calibration calculation, which improves precision of measured output variables.

As a result, influences on the calibration of different tissue properties can be minimized in order to measure arterial or venous or the combination of arterial and venous oxygenation. It has been discovered that by choosing one of the wavelengths as a geometric mean value of two other wavelengths, variations due to scattering can be reduced. Additional determination of light attenuation can reduce measurement errors because of variations of light absorption due to different tissue composition, i.e., variations of relative amounts of muscle, skin, fat, bone, etc.

It is noted that as used in the present specification, “venous” and “mixed venous” are synonyms, “attenuation” refers to absolute or differential attenuation, “tissue oxygenation” refers to arterial, mixed venous, or venous oxygenation or a combination thereof, and the phrase “about” in reference to wavelengths quantifies in a range of +/−80 nm, and in reference to distance quantifies in a range of +/−2 cm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing changes of light absorption by blood over time;

FIG. 2 is a graph illustrating the dependency of arterial oxygen saturation on the measurement variable R for different optical tissue properties;

FIG. 3 shows a reflectance oximetry sensor according to the invention in schematic cross-section;

FIG. 4 shows a finger clip sensor according to the invention in schematic cross-section;

FIG. 5 is a diagram of a multidimensional calibration of oxygenation for the two measuring variables R1, R2 vs. SaO₂;

FIG. 6 is a schematic diagram of an oximetry system in operation;

FIG. 7 is a side view of a fetal scalp sensor according to the invention;

FIG. 8 is a bottom view of the sensor of FIG. 7;

FIG. 9 is a bottom view of the sensor of FIG. 3;

FIG. 10 is a side cross-sectional view of a variation of the sensor of FIG. 3;

FIG. 11 is a side cross-sectional view of another variation of the sensor of FIG. 3;

FIG. 12 is a bottom view of the sensor of FIG. 11;

FIGS. 13-14 are side cross-sectional views of reflectance sensors fixed on the forehead;

FIG. 15 shows a system for determining cardiac output;

FIG. 16 shows person with wrist worn display and sensor applications on different sites of the body;

FIG. 17 is a schematic diagram of a hardware processing unit for an oximetry system according to the invention;

FIG. 18 is a diagram of a multidimensional calibration of oxygenation for the two measuring variables Rv1, Rv2 vs. SvO₂; and

FIG. 19 is a flow chart illustrating signal processing flow for a model-based determination of oxygen in blood.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The diagram of FIG. 1 shows the fundamental effect on which pulse oximetry and comparable methods to determine arterial blood oxygenation are based. When measuring light absorption of tissue in vivo light absorption changes synchronously with every heart cycle. The diagram illustrates the change of light absorption versus time, which is caused by arterial pulsations that can be measured while systole and diastole. During systole and diastole the pressure on the arterial vessel system varies from 80 mmHg to 120 mmHg. The change of light absorption is called the AC-signal. The DC-signal, the time-invariant part of light absorption, is caused by the non-pulsating part of the arterial blood, the venous blood, bone, hair, tissue and other constant absorbing constituents versus time. The time-invariant signal is the basis for the calculation of the mixed venous oxygenation of tissue; thus, a major part of the absorption is caused by venous blood and a minor part by arterial blood.

FIG. 2 shows two calibration curves in a diagram with SaO₂ vs. R. Calibration line 42 is only valid for a first distinct set of optical properties. Calibration line 40 is only valid for a second distinct set of optical properties. The valid set of optical properties can be determined by an optical system illustrated in FIGS. 3 and 6 with a sensor 31S, which is placed on tissue 46 and connected via a plug 66 to a display device 64. Additionally, FIG. 2 shows two horizontal lines at SaO₂=0.6 and at SaO₂=0.4 and one vertical line at R=1.4. If an optical system determines only R without registering the two different sets of optical properties, this would result in an error of 0.2 SaO₂ (SaO₂ at first set of optical properties—SaO₂ at second set of optical properties). An analogous relation also exists for the mixed venous saturation of blood SvO₂ and a measurement variable Rv1 and Rv2 for mixed venous oxygenation (FIG. 18).

FIG. 3 shows an oximetry sensor 31S on the upper part of the figure which is placed on tissue 46. The sensor 31S contains two light emitters 31E, 32E and two light detectors 31D, 32D. The arrows A1 through A4 show how light passes from emitters to detectors through tissue. A1 stands representative for light which is emitted in emitter 31E and received in detector 31D. A2 is light emitted in emitter 32E and detected in detector 31D. A3 is light emitted in 31E and received in 32D and A4 is light emitted in emitter 32E and detected in detector 32D.

FIG. 4 shows a finger clip sensor 54 which is fixed on a finger 48. The finger clip sensor incorporates emitters 31E, 32E and detectors 31D, 32D. The electrical sensor signals of the finger clip sensor are transmitted via a sensor cable 60. The signals can also be conveniently transmitted wirelessly by means well known in the art (not shown).

FIG. 5 illustrates a multidimensional calibration of SaO₂ vs. R1 and R2. A certain combination of R1 and R2 corresponds to a data point on the calibration plane, which indicates the saturation level SaO₂. An analogous relation also exists in FIG. 18 for the mixed venous saturation of blood SvO₂ and two related measurement variables Rv1 and Rv2 for mixed venous oxygenation.

FIGS. 7 and 8 show a fetal scalp sensor 74 with a set of emitters 31E, 32E, 33E and 34E and a set of detectors 31D, 32D, 33D and 34D from side and bottom views, respectively. The sensor can be fixed on the scalp of the fetus via a spiral needle 76 during labor. Additionally, an electrocardiogram (ECG) of the fetus can be transmitted via the needle 76.

FIG. 9 is a bottom view of sensor 31S from FIG. 3. Detectors 35D and 36D have a concentric form to maximize reception of light emitted by the emitters 31E and 32E.

FIGS. 10-12 show several modifications of sensor 31S. FIG. 10 shows sensor in side view with a flat body where detectors 31D, 32D and the emitter 32D are grouped close together and emitter 32E is positioned far from this group. The sensor can be fixed via a band 108 on tissue. A light shield 110 minimizes the influence of ambient light.

FIG. 11 shows a sensor with a sensor holder 122, while FIG. 12 is a bottom view of sensor of FIG. 11. The bottom side of sensor holder 122 can be covered with medical glue or adhesive. If sensor holder 122 is placed on sensor 31S according to FIG. 11 and applied to tissue 46, fixation is possible by glue on sensor holder 122. Sensor holder 122 can thus be constructed in an inexpensive and disposable manner. Alternatively, the bottom side of the sensor, which is applied to tissue, can be directly covered with glue. The disadvantage of this is that the sensor can not be reused. The heart rate is detected via ECG-electrode 123 which contacts the skin.

FIGS. 13 and 14 show two variations of sensor 32S applied on the forehead of a person. In the first variation shown in FIG. 13, sensor 32S is fixed via a band 108 to the forehead. The arrows A32 and A42, which represent how light travels from the emitters 31E, 32E to the detectors 31 and 32D, pass through forehead tissue 152 and bone of skull 150 and pass or touch brain 148. The arrows A12 and A22 only pass through forehead tissue 152 and bone of skull 150.

The second variation of sensor 32S also applied on the forehead is shown in FIG. 14. The arrows A11, A21, A31 and A41 compared with arrows A12, A22, A32 and A42 of FIG. 13 show that by variation of the position of light detectors and emitters, oxygen content can be sensed differently without changing the outline of the sensor variation used.

FIG. 15 shows a patient lying on a bed being supplied with oxygen by an intubation tube 210, and an anaesthesia machine 204. The anaesthesia machine 204 is connected to the patient and has an inventive device for measuring oxygen consumption or carbon dioxide production of the patient. The sensor 32S is placed on the forehead of the patient, and is connected with oxygen extraction monitoring device 206, which calculates SaO₂ and SvO₂and oxygen extraction. The monitoring device 206 and the anaesthesia machine 204 are linked to a third device 202, which calculates cardiac output or trend of cardiac output.

FIG. 16 illustrates the use of oxygen monitoring at different application sites for sports activity, in which a wrist worn display device 220 can receive oxygenation data from a forehead-band-sensor 214, from a chest-band-sensor 224, from an arm-band-sensor 218 or from a finger-glove-sensor 222.

FIG. 17 shows the hardware for evaluating oxygenation by using two emitters 31E and 32E and two detectors 31D and 32D. The LED-drive 226 energizes the two emitters via lines 238, 248 which can incorporate coding hardware, to adjust calibration for the multidimensional calibration or to adjust calibration for varying emitter detector geometry. The amplifiers AMP1 232 and AMP2 234 are connected to detectors 31D and 32D. The demultiplexer DEMUX 320 selects each wavelength used in every emitter timed synchronously according to the switching state of the LED-DRIVE 226 and delivers the measured data via an AD-Converter AD-CONV. 236 to the CPU 228.

FIG. 19 illustrates the signal flow of a model-based calibration. An input processing circuit 260 is the first part of the signal flow. The processing circuit is connected with a circuit for calculating light attenuations 262 and a circuit calculating different measurement variables 264. The calculation for light attenuations 262 is a basis for a model-based determination circuit for mixed venous oxygenation 266 with a joint circuit to output a value for the mixed venous oxygenation SvO₂ 270. A model-based determination circuit for arterial oxygenation 268 is connected to the circuit for calculating light attenuations 262 and the circuit calculating different measurement variables 264. The output value for a arterial oxygenation circuit for SaO₂ 272 is linked to the model-based calculation for SaO₂ 268.

By using three instead of two wavelengths to measure the arterial oxygenation, the following approximation can be derived with the help of diffusion theory. The result of this operation is:

$\begin{matrix} {R^{\prime} = {{\frac{{{Rw}\; 2},{w\; 1}}{{{Rw}\; 1},{w\; 0}}*\frac{{LAw}\; 2*{LAw}\; 0}{{LAw}\; 1*{LAw}\; 1}} + Q}} & (2) \end{matrix}$

where Rw2,w1 and Rw1,w0 are calculated according to equation (1) using wavelengths w1, w1, and w2 and Q is a correction parameter.

Light attenuation LAwx can be calculated in the following or similar manner: LAwx=1n(Iwx/Iwxo)

LAwx corresponds to the logarithm of the ratio of light intensity Iwxo which is the emitted and light intensity Iwx the received light passing through tissue at wavelength wx. The index following suffix wx indicates the selected wavelength.

Graaff et al showed that scattering in tissue decreases for higher wavelengths according to exponential functions (see: Applied Optics; Reduced Light-Scattering Properties for Mixtures of Spherical Particles: A Simple Approximation Derived from Mie Calculations by R. Graaff; 1992). Absorption variation may also be taken from other measures or approximations such as the ac/dc ratio. The amplitude may be any measure such as peak-to-peak, RMS, average, or cross correlation coefficient. It may also be derived from other techniques such as Kalman filtering or a measure of the time derivative of the signal. Also, while calculations utilizing ratios of absorptions at different wavelengths are shown, alternate calculations may be used to give the same or approximately the same results. For instance the absorptions could be used directly, without calculating the ratios.

A preferred selection of the wavelengths combination to reduce the influence of scattering is defined by the following equation, with wavelength w1 as the geometrical mean value of wavelength w0 and wavelength w2, defined as: w1=√(w0*w2) This combination minimizes the variation band of correction parameter Q, which has a default value of about one. The measurement variable R′ of equation (2) has minimized error related to variation of scattering and blood content of tissue.

EXAMPLES EXAMPLE 1

The sensor 31S shown in FIG. 3 is used to determine the arterial oxygenation and the mixed venous blood oxygenation of tissue with improved precision. Equation (2) is used to provide a measurement variable R′ for the arterial oxygenation. For each of the emitters 31E and 32E, three wavelengths are defined. Initially, two measurement wavelengths w0=940 nm and w2=660 nm are selected. Using equation (4) the third wavelengths w1 is about 788 nm. Wavelength w2=805 nm is chosen because it is close to the calculated third wavelength and is additionally at an isobestic point of the blood absorption spectrum. The next step is to determine the resulting light attenuation LA for each of the three wavelengths w0, w1 and w3: LAw1=LA(A3w1)+LA(A2w1)−LA(Alw1)−LA(A4w1)  (5) LAw2=LA(A3w2)+LA(A2w2)−LA(Alw2)−LA(A4w2)  (6) LAw3=LA(A3w3)+LA(A2w3)−LA(Alw3)−LA(A4w3)  (7)

where LA(Axwy) is the logarithm of received light intensity in the detector related to light arrow Ax at wavelength wy. Each LA(Axwy) here is weighted with the factor 1. The suffix x for light arrows Ax represents the number of the selected light arrow and y the suffix for the selected wavelength. Instead of the logarithm of light intensities, light intensity itself can be used in (5)-(7) and “+” is replaced by “*” and “−” is replaced by “/”.

In the next step, Rw2,w1 and Rw1,w0 are calculated according to equation (1). As a result R′ can be determined using equation (2) with Q as a correction factor which can be dependant on Rw2,w1 or Rw1,w0. The measured arterial oxygenation which is dependant on R′ has minimized influence of scattering, blood content or other optical absorbing constituents in tissue.

The quotient in (8) which is part of (2) delivers a measurement variable Rv′:

$\begin{matrix} {{Rv}^{\prime} = \frac{{LAw}\; 2*{LAw}\; 0}{{LAw}\; 1*{LAw}\; 1}} & (8) \end{matrix}$ Rv′ is a measure of optical absorption of tissue with decreased influence of scattering. Therefore it can be used as a signal for mixed venous oxygenation SvO₂.

A mathematically identical form of (2) is:

$\begin{matrix} {{R^{\prime} = {{Rw}\; 2}},{{w\; 0*\frac{{Rv}^{\prime}}{{{Rw}\; 1},{w\; 0*{Rw}\; 1},{w\; 0}}} + Q}} & (9) \end{matrix}$

According to (9) the following equation can also be used to determine a measurement variable R1′ for SaO₂:

$\begin{matrix} {{{R\; 1^{\prime}} = {{Rw}\; 2}},{w\; 0*{f\left( {\frac{1}{{{Rw}\; 1},{w\; 0*{Rw}\; 1},{w\; 0}},{Rv}^{\prime},Q} \right)}}} & (10) \end{matrix}$

where f is an empirical function of optical tissue parameters with variables defined above.

An empirical calibration which reduces influence of absorption and scattering of tissue on the measured variables with the variables LAw1, LAw2, LAw3, Rw1,w2 and Rw2,w3 for the whole saturation range of blood is complex. An pure empirical calibration based on these parameters additionally for different application sites is probably impossible. The proposed model-based method reduces complexity of calibration. SaO₂ can be determined with improved accuracy being only dependent on R′.

It is also possible to use this method for other light absorbing or scattering constituents of blood like carboxyhemoglobin, methemoglobin, bilirubin or glucose dissolved in blood. Light wavelength in the range from 600 nm—1000 nm can be used for carboxyhemoglobin and methemoglobin. Glucose shows an absorption peek dissolved in blood at 1100 nm and bilirubin in the lower wavelengths range from 300 nm—800 nm. For every additional constituent an additional wavelengths has to be chosen. That means that to measure SaO₂ and methemoglobin at a time, four wavelength have to be selected and two different measurement variables R′1 and R′2 according equation (9) have to be defined. Accordingly, the resulting output for SaO₂ is dependent on R′1 and methemoglobin on R′2.

As a result sensor 31S is able to measure arterial and mixed venous oxygenation and other blood constituents at a time with reduced influence of measurement errors due to scattering and absorption of tissue.

EXAMPLE 2

In FIG. 4 finger clip sensor 54 is shown with the two emitters 31E, 32E and the two detectors 31D and 32D. The benefit of the finger clip sensor is that it is easy to apply. Equivalent to sensor 31S in FIG. 3, four representative light paths between the two emitters and the two detectors are possible so that all calculations according example 1 can be performed in order to calculate the output variables R′ and Rv′ as a measure for mixed venous and arterial oxygenation in the finger 48. The corresponding calculations can also be performed using sensor of FIG. 9. The difference here is the alternative form of detectors 35D and 36D, which are able to increase detected light intensity due to an enlarged, concentric detector area.

EXAMPLE 3

FIG. 5 shows a multidimensional calibration of SaO₂ vs. R1 and R2. R1 and R2 can be calculated according (1) by selecting two wavelengths pairs where for the first wavelengths pair the wavelengths wm1=660 nm and wm2=910 nm is chosen and for the second wavelengths pair wm3=810 nm and wm2=910 nm. The second wavelengths pair is less sensitive towards arterial oxygenation and is used to compensate errors due to optical tissue parameter variations. In order to guarantee that the multidimensional calibration delivers improved precision in presence of varying tissue parameters, it is important to select exactly the correspondent calibration which is specified for a distinct wavelengths set and a distinct detector emitter distance. Therefore additional information has to be coded to the selected sensor. The tissue oximeter device can read out this information and use the appropriate calibration. The coding of information can be achieved for example by a resistor implemented in the LED drive line of the sensor (see FIG. 17: 248, 238).

A variant of a multidimensional calibration (FIG. 5) can be achieved by calculating R1 according to equation (2) and R2 according to equation (8). This minimizes the error of displayed arterial oxygenation SaO₂ due to varying optical tissue absorption.

EXAMPLE 4

In FIG. 7 a fetal pulse oximetry sensor 74 is shown, which punctures the skin on the head of the fetus with a spiral needle 76. The bottom view of FIG. 8 shows sensor 74 with 4 emitters 31E, 32E, 33E, 34E and four detectors 31D, 32D, 33D, 34D. Apparently, more than four different light paths per selected wavelength between emitters and detectors are possible. This additional information is used to calculate a whole set of resulting light attenuations LAx. For the different light paths it is also possible to compute a set of measurement variables Rx. Generating a mean weighted value (weight can depend on the noise of the related measurement signals) LAm and Rm of the variables LAx and Rx helps to reduce errors due to tissue inhomogenities. To achieve a stable measure for the optical tissue parameters, which are not influenced by locally varying tissue compositions, is important to minimize errors to precisely determine the inputs of model-based parameters.

EXAMPLE 5

A brain oximeter is shown in FIG. 13 which is positioned on the right side of the forehead of a patient. The cross section of the brain illustrates how four light paths travel through tissue from emitters 31E, 32E to the detectors 31D and 32D, representative for one wavelength. A resulting light attenuation LA can be achieved for each wavelength by adding light attenuations of A32 and A22 and subtracting therefrom the light attentions which are related to A42 and A12. The resulting light attenuation LA is then independent on dirt on emitters or detectors or on degeneration of those parts, which is an important feature since those sensors can be reused. Three wavelengths are chosen for each of the two emitters 31E and 32E of the sensor in FIG. 13 of the brain oxymeter: wb1=660 nm, wb2=740 nm and wb3=810 nm.

The ratio Rvb of the resulting light attentions LAwb2 and LAwb3 is used as a measure for the mixed venous oxygenation. The resulting light attenuation at wavelength wb3=810 nm can be used to eliminate the dependency of blood content in tissue of Rvb with a multidimensional calibration of SvO2 vs. Rvb and LAwb3.

A preferred emitter-detector distance between emitter 32E and detector 31D is greater than 2 cm. The longer the emitter-detector distance is, the deeper the penetration depth into the brain. In order to achieve maximum penetration depth at a minimum of sensor outline, the distance between an emitter and a detector should be the maximum distance between all emitters and detectors. In various examples, the emitter-detector distance is either about 4 cm, at least 4 cm, about 2 cm, or less than 2 cm.

FIG. 14 shows an example where within the sensor, the two detectors have the maximum distance and the detector and emitter elements are grouped symmetrically with regard to the center of the sensor. The resulting maximum penetration depth of light path A31, A21 is here less than maximum penetration depth of light path A32 of the sensor which illustrated in FIG. 13 because the maximum emitter detector distance is also less compared to sensor in FIG. 13 at the same total outline of the sensors. Positioning emitters and detectors asymmetrically is therefore the best choice to achieve oxygenation measurements in deep layers of tissue.

FIG. 12 shows a bottom view of a brain oximetry sensor, in which emitter 31E and detectors 31D and 32D are positioned in a triangle. The light paths between emitter 31E and 31D and between 31E and 32D using the wavelengths wb1=660 nm and wb3=810 nm are determined to evaluate the measurement variables Rp1 and Rp2 which are calculated according to equation (1). The mean value of Rp1 and Rp2 is used as the output value for the arterial oxygenation SaO₂. Alternatively, as shown in FIG. 12 a, the emitters 31E and 32E can be positioned where detectors 31D and 32D are located and detectors 31D and 32D are placed at the location of emitter 31E and 32E in FIG. 12.

EXAMPLE 6

Referring to Example 5, a brain oximetry sensor was described which is able to determine arterial and mixed venous oxygenation of tissue. These two parameters can be used to calculate the oxygen extraction of tissue. A measure therefor can be the difference of arterial and mixed venous oxygenation. Oxygen extraction reflects how well tissue is supplied with oxygen, and can additionally be used to calculate the cardiac output or the trend of the cardiac output CaOut non-invasively.

FIG. 15 shows a patient being supplied with air via an intubation tube 210. The oxygen consumption or CO₂ generation is determined within an anaesthesia machine 204. Brain oximetry sensor 32S is connected to SaO₂ and SvO₂ display device 206. The information of device 204 and device 206 is evaluated in a cardiac output monitor 202 in the following or similar manner:

$\begin{matrix} {{CaOut} = \frac{\left( {{oxygen}\mspace{14mu}{consumption}\mspace{14mu}{per}\mspace{14mu}{time}} \right)}{{SaO}_{2} - {SvO}_{2}}} & (11) \end{matrix}$

EXAMPLE 7

Knowledge of oxygenation of tissue of parts of the body is of high interest for sports activity monitoring. The oxygenation the muscles of the upper leg or upper arm can reflect the training level for different activities of sport. FIG. 16 shows an athlete wearing various sensors which are connected by a line or wirelessly with a wrist-worn-display 220. A sports activity sensor can have the same topology as the above mentioned brain sensor of FIG. 12. Emitter-detector distances however vary, depending on desired tissue monitoring depth. Preferred wavelengths to monitor the mixed venous oxygenation are ws1=700 nm, ws2=805 nm and ws3=870 nm. A resulting light attenuation LA is calculated for each wavelength: LWws1, LAws2 and LAws3 with ws1, ws2 and ws3 as index for the selected wavelengths. A measurement variable for the mixed venous oxygenation Rvs is obtained in the following or similar manner:

$\begin{matrix} {{Rvs} = \frac{{{LAws}\; 1} - {{LAws}\; 2}}{{{LAws}\; 2} - {{LAws}\; 3}}} & (12) \end{matrix}$

Less influence of light scattering and absorption of tissue can be achieved for the determination of mixed venous oxygenation in this way.

A further improvement for better measurement precision can be achieved by generating an output value for the mixed venous oxygenation which is dependant on a multidimensional calibration of SvO₂ vs. Rvs and Rv.

Although the description above contains many specificities, these should not be constructed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example the shape of the emitters can be rectangular, emitters can include LEDs, detectors photodiodes; the shape of the brain sensor can be round; the proposed methods to calculate arterial and mixed venous oxygenation of tissue can be combined in different combinations, signals can be processed by Kalman filters in order to reduce influence of noise caused by motion or other unwanted sources, etc. 

What is claimed is:
 1. An apparatus for determining blood oxygenation in a tissue, the apparatus comprising: a sensor interface having a first emitter and a second emitter configured to emit light into the tissue and having a first detector and a second detector configured to detect light from the tissue, wherein an emitter-detector distance is selected to provide a light path that travels through the tissue at a depth of penetration that includes a selected region of the tissue; a processor configured to: determine light attenuation LA(A1,wsj) of light emitted by the first emitter and received by the first detector; determine light attenuation LA(A4,wsj) of light emitted by the second emitter and received by the second detector; determine light attenuation LA(A2,wsj) of light emitted by the first emitter and received by the second detector; determine light attenuation LA(A3,wsj) of light emitted by the second emitter and received by the first detector; and determine an optical characteristic of said tissue based on a resulting light attenuation LAwsj at said wavelength wsj, the resulting light attenuation determined by summing the light attenuations of LA(A2,wsj) and LA(A3,wsj) and deducting the light attenuations LA(A1,wsj) and LA(A4,wsj), the optical characteristic representative of oxygenation and corresponding to oxygenation of venous blood in tissue of said selected; and an output device coupled to the processor and configured to receive the optical characteristic.
 2. The apparatus of claim 1, wherein said sensor interface is configured to be coupled to the tissue by at least one of a fixation band, a sensor holder, medical glue, a finger glove, and a finger clip.
 3. The apparatus of claim 2, further including a battery coupled to at least one of the sensor interface, the processor, and the output device.
 4. The apparatus of claim 1, wherein said output device includes a wrist worn display.
 5. The apparatus of claim 4, wherein at least one of the first emitter and the second emitter includes an LED.
 6. The apparatus of claim 4, wherein at least one of the first detector and the second detector includes a photodiode detector.
 7. The apparatus of claim 1 wherein the processor is configured to repeatedly determine light attenuations and repeatedly determine the optical characteristic.
 8. The apparatus of claim 1 wherein the processor is configured to determine a weight for at least one of light attenuation LA(A1,wsj), light attenuation LA(A2,wsj), light attenuation LA(A3,wsj), and light attenuation LA(A4,wsj), wherein the weight is selected to reduce an error due to tissue inhomogeneity.
 9. The apparatus of claim 1 wherein the processor is configured to determine light attenuation using a logarithm of light intensity.
 10. Apparatus for measuring tissue oxygenation comprising: a sensor interface adapted to be coupled to a tissue site and including at least two emitters placed apart from each other on a surface of said interface and configured to emit light of at least two wavelengths into tissue and including at least two detectors for detecting light passing through the tissue on a plurality of paths and wherein a selected path of the plurality of paths is configured to provide a depth of penetration that travels through a selected region of the tissue; means for determining at least two signals based on detected light corresponding to wavelengths wsj and using said two detectors and said two emitters, wherein the at least two signals are determined by adding light attenuations corresponding to at least a first pair of selected paths of the plurality of paths and deducting light attenuations corresponding to at least a second pair of selected paths; and means for generating a first output representative of oxygenation and corresponding to oxygenation of venous blood in tissue of said selected region, the first output based on the at least said two signals.
 11. The apparatus of claim 10, wherein said means for determining is configured to: determine light attenuation LA(A1,wsj) of light emitted by a first emitter of the at least two emitters and received at a first detector of the at least two detectors; determine light attenuation LA(A4,wsj) of light emitted by a second emitter of the at least two emitters and received at a second detector of the at least two detectors; determine light attenuation LA(A2,wsj) of light emitted by said first emitter and received at said second detector; determine light attenuation LA(A3,wsj) of light emitted by said second emitter and received at said first detector; and wherein adding light attenuations corresponding to at least a first pair of selected paths of the plurality of paths includes summing the light attenuations of LA(A2,wsj) and LA(A3,wsj) and wherein deducting light attenuations corresponding to at least a second pair of selected paths includes subtracting attenuations LA(A1,wsj) and LA(A4,wsj).
 12. The apparatus of claim 11, wherein the sensor interface is configured to couple with the tissue site at a position near a brain.
 13. The apparatus of claim 11 wherein the means for determining is configured to repeatedly determine the two signals.
 14. The apparatus of claim 10, wherein said at least two emitters includes a first emitter and a second emitter and said at least two detectors includes a first detector and a second detector and further wherein a first distance between the first emitter and the first detector differs from a second distance between the second emitter and the second detector.
 15. The apparatus of claim 10, wherein a first distance between a first emitter of said at least two emitters and a first detector of said at least two detectors is about 4 cm.
 16. The apparatus of claim 10, further including means for generating a second output representative of arterial oxygen saturation based on a path between a first emitter of said at least two emitters and a first detector of said at least two detectors.
 17. The apparatus of claim 10, wherein a first emitter of said at least two emitters is configured to emit at a wavelength of about wb1=740 nm and wherein a second emitter of said at least two emitters is configured to emit at a wavelength of about wb2=805 nm.
 18. The apparatus of claim 10, wherein at least one emitter of said at least two emitters is configured to emit at a wavelength about wb3=660 nm and wherein the means for generating is additionally configured for generating a second output corresponding to arterial oxygenation.
 19. The apparatus of claim 10, wherein at least one emitter of said at least two emitters includes a light emitting diode (LED).
 20. The apparatus of claim 10, further including a means for calculating coupled to the means for determining, the means for calculating configured to reduce dependency of said first output on Hb and blood content.
 21. The apparatus of claim 10, further including an affixation structure coupled to said sensor interface, said affixation structure including at least one of medical glue, a fixation band, or a disposable sensor holder.
 22. The apparatus of claim 10, wherein the sensor interface is configured to couple with the tissue site at a position near a brain.
 23. The apparatus of claim 10, wherein a first distance between a first emitter of said at least two emitters and a first detector of said at least two detectors is less than 2 cm.
 24. Apparatus for measuring oxygenation comprising: a sensor interface adapted to be coupled to a tissue site and including at least two emitters placed apart from each other on a surface of said interface and configured to emit light of at least two wavelengths into tissue and including at least two detectors including a first detector and a second detector, wherein the first detector is configured to generate a first output signal corresponding to detected light of the at least two wavelengths passing through the tissue along a first plurality of paths and wherein the second detector is configured to generate a second output signal corresponding to detected light of the at least two wavelengths passing through the tissue along a second plurality of paths, and wherein a first distance between a first emitter of the at least one emitter and the first detector is at least 4 cm, wherein at least one path is configured to provide a depth of penetration of a light path that travels in a selected region of the tissue; and means for determining oxygenation corresponding to venous blood in the selected region of the tissue corresponding to the first output signal and the second output signal.
 25. The apparatus of claim 24 wherein said first distance is configured to emit light corresponding to a path that passes through cerebral tissue.
 26. The apparatus of claim 24 wherein said at least two emitters includes at least one light emitting diode (LED).
 27. The apparatus of claim 24 wherein a second distance between a second emitter of the at least one emitter and the second detector is about 2 cm.
 28. The apparatus of claim 24 wherein the means for determining oxygenation is configured to determine oxygenation based on a plurality of light attenuations corresponding to a plurality of output signals, each light attenuation corresponding to a selected path of the plurality of paths, wherein a first pair of light attenuations are added and a second pair of light attenuations are deducted.
 29. The apparatus of claim 24 wherein the at least two emitters are configured to emit light of at least 3 wavelengths.
 30. The apparatus of claim 24 wherein the sensor interface includes a memory configured to store a value readable by a processor. 